1. Field of the Invention
The present invention relates to an electrode, to an electrochemical cell utilizing the electrode and to electrolyzer apparatus which can embody the cell. The electrode has a rigid core with, at least in part, a resilient surface. The electrode, also at least in part, is multi-layered and porous. A representative electrochemical cell utilizing the electrode can include the electrode as anode, cathode or both. Such a cell can be useful in reactions such as metal ion oxidation or reduction, or organic reactions, including destruction of organic species. The electrolyzer apparatus which may embody the cell may have special structure for separating anolyte from catholyte, and the electrolyzer can include electrolyte recirculation. Usually, the cell will be embodied in the electrolyzer in a monopolar format, although the present invention is not limited to the cell in any particular arrangement.
2. Description of the Related Art
It has been known to provide electrode structures in grid form, which can include pieces of expanded metal. The expanded metal pieces can be layered in sheets to provide a form like a laminate. For example, in British Patent No. 1,268,182, there is disclosed the layering of sheets of expanded metal. An electrode of two to four sheets is taught, with an exemplary four sheet expanded copper mesh cathode used with a two sheet expanded titanium mesh anode. The sheets may be of differing surface area, such as a sheet of smaller mesh sandwiched between two outer sheets of larger mesh. For forming a composite electrode, a central solid plate can be utilized, preferably functioning as a bipolar electrode.
Where a very short stack of two expanded metal plates is utilized, this can form a planar electrode. However, with a short stack, as taught in U.S. Pat. No. 4,097,347, a curved, cylindrical electrode may also be provided. This has been taught to be useful, when employed as a cathode, for the electrolytic recovery of gold.
In an article of Pletcher et al, Journal of Applied Electrochemistry, No. 24 (1994), pages 95-106, there are disclosed three dimensional nickel electrodes. They are more particularly depicted as a two stack twin grid from expanded nickel, stacked nets from fine nickel mesh, and a stack of four nickel grids. These structures were chosen so as to conform to the available electrode space of a cell of standard configuration. The electrodes filled an electrolyte compartment of a parallel plate, laboratory-sized electrochemical reactor. These various arrangements were found to be useful in testing the oxidation of alcohols.
For utilization in a commercial operation, there has been taught the layering of electrodes which can be superimposed expanded sheets. Thus, it is disclosed in U.S. Pat. No. 4,828,653 the usefulness of three electrode layers, which layers can vary, typically by geometry, so that the current density of each individual layer is substantially the same. Such a layered structure has serviceability as an anode for high-speed electrogalvanizing processes.
Layering for electrode structure, again with non-uniformity of layers, has also been discussed in U.S. Pat. No. 4,761,216. Therein there is disclosed a four layer electrode having a first layer support plate, a second layer of woven screen mesh, or alternatively, fibers such as stainless steel fibers, a third layer required to be stainless steel fibers and a fourth layer of a mesh wire cloth. This structure is discussed as being useful in an electrochemical membrane cell.
Where electrode layers can be stacked wire screens, the utility of up to 12 nested screens has been disclosed in U.S. Pat. No. 4,224,129. Such a construction, employed in a flow-through design, is taught as being useful for recovery of product from constituents in an electrolyte.
It would nevertheless still be desirable to provide a layered electrode where caution, as in nesting and stacking, can be obviated. It would be desirable to provide such a layered electrode having ease and economy of manufacture, e.g., as by avoiding the alignment and "same size" considerations of stacked sheets or the fabrication of stacks of sheets of differing geometry. It would also be desirable to provide such a layered electrode of simplistic construction avoiding the uniting of fibers and meshes while still providing economy of manufacture coupled with providing a great many layers, which multitude of layers could be selected within a wide range of layers, yet maintain ease and economy of manufacture regardless of selection.
Especially when dilute solutions are used as electrolyte, economic usage of cells for electrochemical processes can be limited because of the difficulty encountered in achieving a high rate of transfer to electrode surfaces. One dilute solution of interest is an electrolyte that is obtained as a scrubbing solution for removing oxides of nitrogen and sulfur from flue gases. The solution uses a ferrous chelate which is oxidized to an inactive ferric state. For re-use, this solution may be processed in an electrochemical cell. One approach for this solution of particular interest utilizes a plurality of cathode and anode compartments separated by ion transfer membranes. Thus, it has been taught in U.S. Pat. No. 4,126,529 to pass a spent scrubbing solution through the cathode compartments of an electrochemical cell. The cathodes are discussed as being a combination of a plate and wire mesh. The regeneration process not only provides for the reduction of the nonreactive ferric chelate to the reactive ferrous chelate, but also can involve the removal of sulfate ions from the scrubbing solution through the ion transfer membranes.
A recent development in regeneration of this electrolyte of particular interest has focused, on the one hand, to maintaining the cell membrane, while on the other hand, enhancing electrode stability. Thus, it has been shown in U.S. Pat. No. 5,320,816 that an otherwise unstable nickel or stainless steel anode material, is stable in the regeneration cell when a pH greater than 12 is maintained in the anode compartment.
It would, however, be desirable to provide for efficient spent scrubbing solution regeneration without resort to special electrolyte control. It would be also desirable to achieve efficient regeneration while providing for ease and economy of cell operation and materials.
Where electrochemical cells embody a multitude of electrodes which might be useful for enhancing the rate for any mass transport limited electrochemical reaction, it has been known to space a multitude of anodes and cathodes within a cell box. For example, in U.S. Pat. No. 4,399,020 there has been disclosed a cell box containing a plurality of anodes and cathodes where the electrolyte flows through the box, which flow can include flow through a reticulate cathode. The reticulate cathodes for such an electrolyzer assembly may be metal foam cathodes. They may be provided with a porous plate support. Thus, it has been shown in U.S. Pat. No. 4,515,672 to provide metallic foam cathodes on one or both sides of a porous support plate. These cathodes can then be employed in a flow-through electrolyzer cell box assembly.
It would still be desirable to provide an electrolyzer cell box comprising enhanced operation of a mass transport limited electrochemical reaction which can be coupled with independent anolyte and catholyte flow. Or to provide such a cell having economy of construction and operation, including economy of recycling electrolyte.